1. “sitting up straight in church”—or maintaining good posture wherever you are.

2. bend down, pick up a something weighted(medicine ball, hand weight, kettlebell).  if using medicine ball, reach back overhead and throw it to your trainer. if using a hand weight lift it overhead and back down to ground bending knees.  if using a kettlebell swing it forward above head and back down to ground again—keep knees soft and bend as approach ground.

3. roll-out,  with knees on floor and elbows on resistance ball slowly roll the ball forward while bracing abdominals while maintaining neutral spine.  trainer can help check this position. 

4. pike, in push-up position with feet on ball, brace abdominals while lifting hips upward, maintaining neutral spine.  trainer can help maintain stability.

the above exercises range from easy to difficult.  easy to difficult should be the order in which you train the abdominals.  never begin with an exercise that is too hard; instead work up to it gaining strength and stability as you go.